Many materials that are normally encountered and exist in the gas phase under standard conditions (e.g. 273-298K, 1 atm) can be liquefied by cooling them to very low temperatures, typically below 200K. These materials include, but are not limited to, the noble gases including helium, neon, argon and krypton, diatomic gases such as hydrogen, nitrogen and oxygen, and hydrocarbon gases such as methane (natural gas), ethane and propane. Of course, this is not an exhaustive list. The liquid forms of these materials are referred to as cryogenic liquids because they must be maintained at cryogenic temperatures, typically below 200K (although certain materials such as propane have somewhat higher boiling points), to keep them in the liquid phase. Even when these materials will be used in the gas phase, it is still advantageous to store and transport them in the liquid phase because a substantially greater mass of material can be stored in a given volume due to the significantly greater density of the cryogenic liquid compared to the gas phase.
Cooling and maintaining these materials at cryogenic temperatures is energy intensive. That is, it requires a great deal of energy to refrigerate cryogenic materials due to the abundant sources of thermal energy that typically surround cryogenic-storage vessels. On Earth, the ambient environment alone, consisting of air typically ranging in temperature from 273K to 300K, provides an infinite source of thermal energy to raise the temperature of a cryogenic liquid inside a storage vessel. Other devices and machines often installed in proximity to cryogenic-storage vessels, e.g. motors and electrical circuits, provide additional sources of thermal energy. The sun provides yet a third source of thermal energy. Thermal energy from all these sources tends to enter a cryogenic storage vessel to raise the temperature of the stored cryogenic material, driven by the temperature gradient between the energy source and the liquid cryogen.
To counteract the natural tendency of thermal energy to enter the cryogenic-storage vessel from surrounding sources, often termed “heat leak,” such storage vessels are typically encapsulated by insulation. The insulation is a material or combination of materials that exhibits a higher resistance to heat transfer than the storage-vessel wall alone. Thus the insulation slows the rate of heat leak into the storage vessel, thus reducing the cooling duty required to maintain the vessel and its contents at the desired cryogenic temperature. Alternatively, in the absence of an active cooling system the insulation extends the length of time the cryogen may be stored in the liquid state, or at least minimizes the rate of venting of vaporized cryogenic material to avoid overpressure that may result in rupture of the storage vessel.
In addition to installing insulation material around the storage vessel, it is also common to draw a vacuum around the storage vessel and insulation. This removes a significant thermal-transfer medium, air, from the immediate vicinity surrounding the storage vessel. Air is an effective medium for both conductive and convective heat transfer. By drawing a vacuum around the storage vessel and its insulation, these modes of heat transfer into the storage vessel can be minimized, leaving radiation as the principle mode of heat transfer left to combat.
Conventionally, a cryogenic-storage vessel is disposed within the volume of a second, larger storage vessel, which serves as vacuum vessel. A vacuum is drawn on the interior volume of the vacuum vessel, which places the space surrounding the cryogenic-storage vessel under vacuum. The vacuum vessel is conventionally made of steel, such as stainless steel. Such vacuum vessels are bulky and heavy.
For cryogenic materials such as hydrogen and oxygen that are used as propellants in aircraft and other launch vehicles, it is desirable that the entire cryogenic storage system be as lightweight as possible. This permits the system to have the smallest impact on the aerodynamic characteristics and flight performance of the vehicle. At the same time, these systems are in proximity to a very strong heat source: a jet- or rocket-propulsion system. Thus, in addition to being lightweight it is desirable that an airborne cryogenic-storage system inhibit the transmission of thermal energy into the storage vessel as much as possible.